MR imaging method with motion compensation

ABSTRACT

The invention relates to an MR method where the motion of an object to be imaged is examined during a preparation phase preceding the actual MR examination and where the necessary sequences for the subsequent MR examination are modified during such examination in such a manner that the motion is compensated.

[0001] The invention relates to an MR method for the imaging of an atleast partly moving object in an examination zone, wherein the nuclearmagnetization is excited during an MR examination in the presence of asteady magnetic field by sequences which include at least a respectiveRF pulse of defined frequency, a number of MR signals received with adefined phase position and produced under the influence of additionalmagnetic gradient fields is evaluated, a motion parameter of the objectis continuously measured for motion compensation and the parameters ofthe sequence are varied in dependence on the measurement. The inventionalso relates to an MR apparatus for carrying out the method as well asto a computer program for controlling the control unit of an MRapparatus in such a manner that the method can be executed.

[0002] Moving object must be imaged notably in the field of medicalimaging, for example the heart or the coronary vessels. In this respectit is known from a study by Wang et al. (MRM 33:713-719 (1995)) that thedisplacement of the human heart is proportional to therespiration-induced diaphragm motion. Therefore, the cardiac motion canbe at least partly compensated by monitoring the motion of thediaphragm, for example by means of so-called navigator pulses and byvarying the frequency or the phase of the RF pulse or the phase positionof the MR signals in dependence thereon (so-called slice tracking), thusreducing the motion artefacts in the MR image.

[0003] In comparison with a method in which MR signals are acquired onlyduring a given phase of the respiratory motion, the cited method offersthe advantage that the measuring time required for the acquisition ofall MR signals is reduced. It is a drawback that the motion compensationis comparatively inaccurate, because the motion of the heart as afunction of the diaphragm motion differs from one patient to another.

[0004] In order to mitigate this drawback, a publication by Taylor et alin ISMRM, 322 (1998) proposes to derive individual correction factorsfrom MR images that are acquired prior to the actual MR examination, andshow the heart in different respiratory phases. This enables moreaccurate compensation, but the acquisition of the individual correctionfactors necessitates careful evaluation of the previously formed MRimages by the examiner.

[0005] It is an object of the present invention to propose a method ofthe kind set forth such that a comparatively accurate motioncompensation is achieved in a simple manner. This object is achieved inaccordance with the invention by taking the following steps:

[0006] measuring the variation in time of n correlated motion parametersduring a preparation phase preceding the MR examination,

[0007] measuring m of said n motion parameters during the MRexamination,

[0008] deriving the motion parameters that have not been measured duringthe MR examination from the measured motion parameters,

[0009] varying the parameters of the sequence in dependence on thecalculated or measured motion parameters in order to achieve motioncompensation.

[0010] In conformity with the invention the individual adaptation takesplace during a preparation phase that precedes the actual MR examinationn correlated motion parameters are then measured quasi simultaneously.The correlation between the individual motion parameters can be derivedfrom such a measurement. During the subsequent MR examination it ismerely necessary to measure m motion parameters (for example, thediaphragm motion, m=1) wherefrom the other motion parameters can bederived on the basis of the previously determined correlation. Theparameters of the sequence (for example, frequency or phase position ofthe RF pulse or the MR signal) can be varied in dependence thereon insuch a manner that motion compensation is achieved.

[0011] The method in accordance with the invention is executedautomatically and does not require evaluation of previously formed MRimages by an examiner. Even though a large number (n) of motionparameters must be measured for the exact motion compensation, theactual MR examination is not affected thereby, because the acquisitionof such motion parameters takes place during the preceding preparationphase.

[0012] The version of the method that is disclosed in claim 2 offerseven more accurate motion compensation. The reason is that when at leasttwo motion parameters are measured, motions that are more complex than asimple translation can be compensated. For example, it is thus possibleto take into account the fact that the heart is compressed or expandedunder the influence of the diaphragm motion. Such compression orexpansion, but also rotation, however, can be compensated only byvarying, in addition to the frequency and the phase position of the RFpulse or the MR signal, the magnetic gradient fields acting during asequence.

[0013] Claim 3 discloses a preferred possibility for the measurement ofthe motion parameters. Navigator pulses enable the excitation of avolume that is limited in two dimensions, for example a cylindrical rod(pencil beam). For example, when such a navigator pulse is incident onthe diaphragm of a patient, the state of motion thereof, or thepatient's respiration, can be determined by evaluation of the MR signalsreceived after the navigator pulse. The motion of a spatially limitedvolume can thus be accurately measured.

[0014] Claim 4 discloses a version of the method that is suitable forthe imaging of the heart (or the coronary vessels). The heart moves notonly under the influence of respiratory motion, but also because of thecardiac action, a cardiac cycle being significantly shorter than arespiratory cycle. During the late diastole of the cardiac action(briefly before the R wave in the ECG), however, there is a low-motionphase of approximately 100 ms. When the sequences are producedexclusively during such a low motion phase, the cardiac cycle will notintroduce any additional motion artefacts.

[0015] The time necessary for the acquisition of all required MR signalscan be reduced in conformity with claim 5. To this end it is necessaryto utilize sequences whose duration is shorter than that of thelow-motion phase of the heart. However, because this phase is short incomparison with a respiratory cycle, it suffices to measure the motionparameters only once during the relevant cardiac cycle.

[0016] Claim 6 describes an MR apparatus for carrying out the method andclaim 7 discloses a computer program for a control unit controlling theexecution of the method in such an MR apparatus.

[0017] The invention will be described in detail hereinafter withreference to the drawings. Therein:

[0018]FIG. 1 shows an MR apparatus that is suitable for carrying out theinvention,

[0019]FIG. 2 shows the location of the measuring points during theacquisition of the motion parameters,

[0020]FIG. 3 shows the variation in time of the motion at the variousmeasuring points,

[0021]FIG. 4 shows the correlation between said motions,

[0022]FIG. 5 shows a cardiac cycle and the situation of the variousacquisition steps during the cardiac cycle, and shows the variation intime of a sequence during the MR imaging.

[0023] The reference numeral 1 in FIG. 1 denotes a diagrammaticallyrepresented main field magnet which generates, a steady, essentiallyuniform magnetic field B₀ which extends in the z direction in anexamination zone (not shown) and has a strength of, for example 1.5Tesla. The z direction extends in the longitudinal direction of anexamination table (not shown) on which a patient is accommodated duringan examination.

[0024] Also provided is a gradient coil system 2 which includes threecoil systems that are capable of generating gradient magnetic fieldsG_(x), G_(y) and G_(z) which extend in the z direction and have agradient in the x, the y and the z direction, respectively. The currentsfor the gradient coil system 2 are supplied by a gradient amplifier 3.Their variation in time is governed by a respective waveform generator,that is, separately for each direction. The waveform generator 4 iscontrolled by an arithmetic and control unit 5 which calculates thevariation in time of the magnetic gradient fields G_(x), G_(y), G_(z)that is required for a given examination method, and loads thisvariation into the waveform generator 4. During the MR examination thesesignals are read out from the waveform generator 4 and applied to thegradient amplifier 3 which forms the currents required for the gradientcoil system 2 therefrom. The control unit 5 also acts on a work station6 which includes a monitor 7 for the display of MR images. Entries canbe made via a keyboard 8 or an interactive input unit 9.

[0025] The nuclear magnetization in the examination zone can be excitedby RF pulses from an RF coil 10 which is connected to an RF amplifier 11which amplifies the output signals of an RF transmitter 12. In the RFtransmitter 12 the (complex) envelopes of the RF pulses are modulated onthe carrier oscillations that are supplied by an oscillator 13 and whosefrequency corresponds essentially to the Larmor frequency (approximately63 MHz in the case of a main magnetic field of 1.5 Tesla). Thearithmetic and control unit 5 loads the frequency and the phaseposition, or the complex envelope, of the RF pulses into a generator 14which is coupled to the transmitter 12.

[0026] The MR signals generated in the examination zone are picked up bya receiving coil 20 and amplified by an amplifier 21. The amplified MRsignal is demodulated in a quadrature demodulator 22 by 290° mutuallyoffset carrier oscillations of the oscillator 13, resulting in a complexMR signal whose phase can also be controlled by the control unit 5. Thecomplex MR signals are applied to an analog-to-digital converter 23which forms MR data therefrom. The MR data is subjected to variousprocessing steps in an evaluation unit 24, that is, inter alia a Fouriertransformation, in order to form MR images.

[0027]FIG. 2 shows diagrammatically the location of the heart H in thethorax of a patient; the heart should be imaged with as few motionartefacts as possible during an MR examination. Also shown is thediaphragm Z and the pulmonary lobe L. The Figure also shows threemeasuring points in which the motion is measured. The measuring point Pis situated on the diaphragm, that is, to the right of the heart (to theleft in FIG. 2). The measuring point P1 is situated at the upper edge ofthe heart and the measuring point P2 at its lower edge.

[0028] The invention is based on the assumption that therespiration-induced motion of an arbitrary point of the heart can bedescribed by an affine transformation in conformity with the equation

R(r,t)=A(t)·r ₀ +r(t)  (1)

[0029] Therein, R(r,t) is the path traveled by this point during therespiratory motion. A(t) is a global (that is, valid for all points onthe moving object), time-dependent (3×3) transformation matrix thatdescribes the rotation and expansion of the moving object H. Thethree-dimensional vector r₀ defines the position of an arbitraryanatomical point on the object H and r(t) is a vector describing theglobal, time-dependent shift of the object H. The motion of the heart asdescribed by the equation (1) can be compensated (that is, an MR imagecan be formed which shows the moving object as if it has not moved) whenthe parameters of the imaging sequence are modified as follows:

[0030] a) Instead of the magnetic gradient field G(t), involving avector with the time-dependent components G_(x), G_(y), and G_(z) use ismade of a compensating magnetic gradient field G_(c) (t) which iscalculated in conformity with the formula:

G _(c)(t)=(A ⁻¹(t))^(T) ·G(t)  (2)

[0031] Therein, A⁻¹(t))^(T) is the inverse transposed matrix of thematrix A(t). The rotation or expansion of the moving object is thuscompensated.

[0032] b) The phase of the RF magnetic field acting on the examinationzone during the RF pulse is modulated with a phase factor Φ_(s)modulated in conformity with the relation

Φ_(s)(t)=e^(iγu(t))  (3)

[0033] It then holds for u(t) that $\begin{matrix}{{u(t)} = {\int_{t}^{t1}{{G_{C}\left( t^{\prime} \right)}^{T}{r\left( t^{\prime} \right)}{t^{\prime}}}}} & (4)\end{matrix}$

[0034] Therein, G_(c)(t′)^(T) is the transposed vector that occurs dueto the interchanging of the rows and columns of the matrix G(t). Thequantity t₁ denotes the end of an RF pulse.

[0035] c) The MR signal is modulated with a phase factor Φ_(e)(t) inconformity with the relation

Φ_(e)(t)=e^(iγv(t))  (5)

[0036] It then holds for v(t) that: $\begin{matrix}{{v(t)} = {\int_{t1}^{t}{{G_{C}\left( t^{\prime} \right)}^{T}{r\left( t^{\prime} \right)}{t^{\prime}}}}} & (6)\end{matrix}$

[0037] The phase modulation of the RF pulse, or the MR signal, asindicated sub b) and c) changes the frequency and the phase of thesesignals, so that the translation of the object H is compensated for theMR imaging.

[0038] The nine elements of the matrix A(t) and the three components ofthe vector r(t) must be determined during the preparation phasepreceding the actual MR examination. Assuming that the elements of thematrix A(t) are linearly independent of one another, up to 12 motionparameters would have to be measured for this purpose. However, thismeasurement can be significantly simplified by assuming that the objectmoves or expands only in the z direction, that is, in the longitudinaldirection of the patient. The vector r(t) is then simplified as:$\begin{matrix}{{r(t)} = \begin{pmatrix}0 \\0 \\{f(t)}\end{pmatrix}} & (7)\end{matrix}$

[0039] and the matrix A of t then becomes $\begin{matrix}{{A(t)} = \begin{pmatrix}1 & 0 & 0 \\0 & 1 & 0 \\0 & 0 & {b(t)}\end{pmatrix}} & (8)\end{matrix}$

[0040] For this simplified motion model it is merely necessary tomeasure two motion parameters, for example the shift of the upper andthe lower heart edge at the points P₁ and P₂ in FIG. 2.

[0041] Therefore, in the preparation phase the shift of the points, P,P₁ and P₂ in the z direction is measured during a few respiratory cycleswhile utilizing suitable navigator pulses. FIG. 3 shows the variation intime of the shifts d, d₁ and d₂ measured for the points P, P₁ and P₂,respectively. It appears that the displacement of the diaphragm underthe influence of the respiratory motion is greater than that of theupper edge and the lower edge of the heart. The shift d₂ of the loweredge of the heart is larger than the shift d₁ of the upper edge of theheart.

[0042] When the values of d₁ and d₂ determined at given instants areplotted as a function of the value d determined at the same instantduring the recurrent respiratory motion, the dependency of these valuesthat describes the correlation of these values is obtained as shown inFIG. 4. Consequently, during the subsequent MR examination it is nolonger necessary to measure the values d₁ and d₂, but merely thequantity d, that is, the motion of the diaphragm. The associated valuesof d₁ and d₂ are then obtained directly from the correlation shown inFIG. 4 (this correlation can be approximated by a respective straightline). The parameters b(t) and f(t) are then obtained as:$\begin{matrix}{{b(t)} = \frac{{d_{1}(t)} - {d_{2}(t)}}{d_{1o} - d_{2o}}} & (9) \\{{f(t)} = {{d_{1}(t)} - {\frac{{d_{1}(t)} - {d_{2}(t)}}{d_{1o} - d_{2o}}d_{1o}}}} & (10)\end{matrix}$

[0043] Therein, d_(1o) and d_(2o) represent the values d₁ and d₂ at areference point that can be selected at random, for example, at the endof exhalation.

[0044] During the subsequent MR examination merely the parameter d mustbe measured, that is, in addition to the imaging MR signals. Using thecorrelations shown in FIG. 4, the values b(t) and f(t) can then bedetermined in conformity with the equations 9 and 10; the matrix A(t)and the vector r(t) can be derived therefrom in conformity with theequations 7 and 8 and the parameters G_(c)(t), Φ_(s)(t), Φ_(e)(t) can becalculated therefrom in conformity with the equations 2, 3 and 5.

[0045]FIG. 5 shows the situation in time of the imaging sequences duringthe actual MR examination in relation to the variation in time of an ECGsignal (first line of FIG. 5) during a single cardiac cycle. As isindicated by the blocks situated therebelow, the MR imaging operationincludes a so-called T₂ preparation (T₂) which should not be mistakenfor the preparation phase provided in conformity with the invention.

[0046] Subsequently, there is generated a navigator pulse N whichexcites the nuclear magnetization in a pencil-shaped zone above thepoint P (see FIG. 2). The position d of the diaphragm during the cardiaccycle can be derived from the MR signals thus produced; this positionmay be assumed to remain constant during the remainder of the cardiaccycle. Subsequently, therefrom the variations in time ofG_(c)(t),φ_(e)(t) and φ_(s)(t) can be calculated in advance so as to bestored (for example, in the waveform generator 4 of FIG. 1). Prior tothe actual imaging sequences (SI) a regional suppression pulse and a fatsuppression pulse (F) can be generated; however, these steps may also beomitted.

[0047] In order to achieve motion compensation, the parameters in thenext sequence must be changed. The variation in time of the gradientmagnetic field G(t)=(G_(s)(t), G_(r1)(t), G_(r2)(t)) must be changed inconformity with the equation 2 for the value of d determined by means ofthe navigator pulse (N). The phase of the RF oscillation during the RFpulse RF must be modulated with the value Φ_(s) in conformity with theequation 3 and the received MR signal with the phase factor Φ_(e) inconformity with the equation 5.

[0048] Subsequently, during the late phase of the diastole of thecardiac cycle there are three imaging sequences with parameters thathave been modified (as previously described) in dependence on themeasured position d of the diaphragm Z. One of these sequences is shownin FIG. 5.

[0049] The third line of FIG. 5 shows the envelope of an RF pulse RFwhose peaks coincide each time with the positive or the negativepolarity of a slice-selective gradient G_(s) (second line). The RF pulseRF is succeeded by phase encoding by activation of the gradient G, for adefined duration and with a selectable amplitude. Subsequently, an MRsignal is read out by means of two periodic read-out gradients (fifthline and sixth line of FIG. 5) G_(r1) and G_(r2) of increasing amplitudewhich extend perpendicularly to one another and to the gradient G_(s)(seventh line). As a result, the nuclear magnetization in the so-calledk space is read out along a spiral arm. The sequence terminates in thatthe excited nuclear magnetization is dephased by a so-called “spoiler”of the gradient G_(s). Subsequently the next sequence is applied, be itthat the next gradients G_(r1) and G_(r2) have been modified in such amanner that the spiral arm followed in the k space has been offset, forexample, 120° relative to the preceding spiral arm (or is subjected to adifferent phase encoding).

[0050] MR imaging can also be performed by means of other sequences thatenable two-dimensional or three-dimensional imaging, for example bymeans of a so-called 3 DFT sequence.

[0051] Furthermore, it is also possible to apply so-called gating whereonly the MR signals acquired in given phases of the respiration are usedto form an MR image. Motion artefacts remaining despite the motioncompensation can thus be reduced, be it at the expense of a longeracquisition time. However, the prolongation in time is not as long as inthe methods wherein the motion artefacts are to be avoided exclusivelyby gating, that is, without any motion compensation, because MR signalscan be acquired only in a small part of the respiration cycle in such acase.

[0052] The invention can also be used for the imaging of other movingorgans, for example the liver or a kidney or kidneys. Finally, the MRmethod in accordance with the invention can be used not only for theexamination of the human body but also for other biological or technicalsystems.

1. An MR method for the imaging of an at least partly moving object (H)in an examination zone, wherein the nuclear magnetization is excitedduring an examination in the presence of a steady magnetic field bysequences (SI), which include at least a respective RF pulse (RF) ofdefined frequency, a number of MR signals received with a defined phaseposition and produced under the influence of additional magneticgradient fields is evaluated, a motion parameter of the object iscontinuously measured for motion compensation and the parameters of thesequence are varied in dependence on the measurement, characterized inthat the following steps are taken: measuring the variation in time of ncorrelated motion parameters (d, d₁, d₂) during a preparation phasepreceding the MR examination, measuring m of such n motion parameters(d) during the MR examination, deriving the motion parameters (d₁, d₂)that have not been measured during the MR examination from the measuredmotion parameters, varying the parameters of the sequence in dependenceon the calculated or measured motion parameters in order to achievemotion compensation.
 2. An MR method as claimed in claim 1,characterized in that it includes the following steps: deriving at leasttwo motion parameters for a motion direction from the measured motionparameters, compensating motion additionally by varying the magneticgradient fields.
 3. An MR method as claimed in claim 1, characterized inthat navigator pulses are used for the measurement of the motionparameters.
 4. An MR method as claimed in claim 1 for the imaging of theheart, characterized in that the sequences are generated under thecontrol of an ECG.
 5. An MR method as claimed in claim 4, characterizedin that per cardiac cycle a number of successive sequences are generatedafter measurement of the motion parameters.
 6. An MR apparatus forcarrying out the method claimed in claim 1, characterized in that itincludes a magnet (1) for generating a uniform, steady magnetic field,an RF transmitter (12) for generating magnetic RF pulses, a receiver(22) for receiving MR signals, a generator (4) for generating gradientmagnetic fields with gradients that vary differently in time and inspace, an evaluation unit (24) for processing the received MR signals,and a control unit (5) which controls the RF transmitter, the receiver,the generator and the evaluation unit and is programmed in such a mannerthat the following steps are executed: measuring the variation in timeof n correlated motion parameters during a preparation phase precedingthe MR examination, measuring m of such n motion parameters during theMR examination, deriving the motion parameters that have not beenmeasured during the MR examination from the measured motion parameters,varying the parameters of the sequence in dependence on the calculatedor measured motion parameters in order to achieve motion compensation.7. A computer program for a control unit for controlling an MR apparatusfor carrying out the method claimed in claim 1 as follows: measuring thevariation in time of n correlated motion parameters during a preparationphase preceding the MR examination, measuring m of such n motionparameters during the MR examination, deriving the motion parametersthat have not been measured during the MR examination from the measuredmotion parameters, varying the parameters of the sequence in dependenceon the calculated or measured motion parameters in order to achievemotion compensation.